The extractive distillation of different aromatics and non-aromatics contained in hydrocarbon mixtures with an N-substituted morpholine as a selective solvent is used to a significant extent in large-scale plants for the recovery of very pure aromatics.
Reformate gasoline, pyrolysis gasoline or crude coke-oven benzene are particularly used as hydrocarbon mixtures. In addition to various aromatics, these hydrocarbon mixtures also contain nonaromatics, which can include paraffins, cycloparaffins, olefins, diolefins and organic sulphur compounds.
By means of extractive distillation the hydrocarbon mixtures are separated into aromatics, on the one hand, and non-aromatics, on the other hand. To this extent this extractive distillation is suitable both for the recovery of pure aromatics and for the recovery of pure non-aromatics, e.g. olefins and diolefins.
In extractive distillation, the aromatics from the hydrocarbon mixture used are concentrated as the extract with the main quantity of the selective solvent in the sump of the extractive distillation column. The non-aromatics from the hydrocarbon mixture used, on the other hand, are concentrated as the raffinate in the head of the extractive distillation column. However, the result of this is that the raffinate occurring at the head of the extractive distillation column still contains significant solvent residues. This solvent component in the raffinate is largely recovered in these extractive distillation processes in a complex and expensive manner.
In the process described in (DE 40 40 145 A1) by contrast with the other known process, both the extractive distillation and the removal of the solvent residues from the raffinate are performed in a single column. For this purpose, the extractive distillation column comprises an additional top column part and the selective solvent is fed to the extractive distillation column via a solvent feed below this upper part of the column. This solvent introduced flows down over the column trays, and in so doing it entrains the aromatics to the sump of the extractive distillation column. The non-aromatic hydrocarbons from the mixture used, on the other hand, rise in vapor form in the extractive distillation column. The top part of the column above the solvent feed serves to enable the solvent residues to be removed from these non-aromatic hydrocarbons. The process known from DE 40 40 145 A1 has proved to be substantially satisfactory, but is open to improvement.
For example, this known process has disadvantages particularly when the hydrocarbon mixture used has a higher aromatics content, e.g. crude coke-oven benzene having an aromatics content of about 85% of pre-purified benzene having an aromatics content of more than 95%. With hydrocarbon mixtures of this kind, the proportion of non-aromatics is very small in relation to the quantity of selective solvent.
In extractive distillation, the aromatics are entrained to the sump of the extractive distillation column by the selective solvent added from above. The non-aromatics rise, as has already been stated, in vapor form to the head of the extractive distillation column. Individual equilibria occur on the individual trays of the extractive distillation column and are dependent particularly on the temperature and on the concentrations of the substances.
These equilibria are disturbed, in the extractive distillation of the known process by irregularities in the column heating or variations in the predetermined solvent temperature or upon changes in the quantity of solvent supplied. These disturbed equilibria have a significant effect on the extractive distillation of a hydrocarbon mixture which has a very high aromatics content, e.g. prepurified benzene having an aromatics content of more than 95%.
Because of the high aromatics content and the low non-aromatics content, particularly in relation to the quantity of selective solvent used, these fluctuations or disturbances cause the aromatics present in high concentration to be no longer completely entrained into the sump by the selective solvent as a result of disturbed equilibria. In particular, low-boiling benzene, which by comparison with the non-aromatics has a lower boiling temperature, can pass to the top part of the column above the solvent feed. As a result, benzene and other low boiling aromatics can reach the head of the extractive distillation column so that a mixture of aromatics and non-aromatics tends to form at the head of the column. Even a small quantity of aromatics reaching the upper part of the column (above the first solvent feed) because of the above fluctuations results in relatively higher aromatics contents in the raffinate when relatively small quantities of non-aromatics are involved. In addition, the aromatics reaching the column head also have an adverse effect on the aromatics yield.